1. Field of the Invention
This invention relates to photochromic glasses, i.e. to glass compositions which darken on exposure to actinic radiation and fade back to their original, normally colourless, state when no longer exposed.
2. Description of the Prior Art
In our British patent specification No. 1367903, we have described and claimed a range of photochromic glasses comprising at least 17% by weight P.sub.2 O.sub.5 as one of the glass forming components, with silver halide crystals dispersed throughout the glass, the total silver content of the glass being at least 0.05% by weight Ag. The specific glasses disclosed in that Specification are alumino-phosphate glasses comprising not more than 40% by weight SiO.sub.2 and between 9% and 34% by weight Al.sub.2 O.sub.3 as further glass forming components, and at least 10% by weight R.sub.2 O, where R = K, Na or Li. They can also contain up to 19% by weight B.sub.2 O.sub.3 though most of them contain no more than 3% to 7% B.sub.2 O.sub.3 and some contain none at all.
Glasses falling within the scope of British Pat. No. 1367903 are now used in the manufacture of ophthalmic lenses for both sunglasses and prescription spectacles. These alumino-phosphate glasses, like the photochromic borosilicate glasses which are also available on the market, exhibit desirable photochromic properties but have relatively slow responses to exposure to, and removal of, actinic radiation, i.e. slow darkening and fading rates. It is desirable, particularly for ophthalmic purposes, to have glasses with faster responses, particularly a faster fading rate. A rapid fading rate is desirable to aid in adjustment to a rapid decrease in available light, as when a wearer of spectacles with lenses of photochromic glass enters a dimly-lit room.